ABSTRACTCulturesof TrichodesmiumNIBB 1067 were grown in the synthetic medium AQUIL with a range of iron added from none to5 × 10−7MFe for 15 days. Chlorophyll‐a, cell counts, and total cell volume were two or three times higher in medium with 10−7M Fe than with no added Fe. Oxygen production rate per chlorophyll‐awas over 60% higher with higher iron. Increased iron stimulated photosynthesis at all irradiances from about 12–250 μE · m−2· s−1. Nitrogen fixation rate, estimated from acetylene reduction, for 10−7and 10−8M Fe cultures was approximately twice that of the cultures with no added Fe. The range of rates of O2production and N2fixation in cultures at the iron concentrations we used were similar to the rates from natural samples ofTrichodesmiumfrom both the Atlantic, and the Pacific oceans. This similarity may allow this clone to be used, with some caution, for future physiological ecology studies. This study demonstrates the importance of iron to photosynthesis and nitrogen fixation and suggests thatTrichodesmiumplays a central role in the biogeochemical cycles of iron